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Over the past year, my students and I have been studying the adsorption of environmentally relevant PAH (polycyclic aromatic hydrocarbons) to the air/water and oil/water interfaces, using surface tension and several spectroscopic techniques. PAH molecules are naturally found in crude oil, are readily formed as products in incomplete combustion, and are of great concern to human (and other organism) health, due to their toxic and often carcinogenic properties. The ultimate goals of these studies are to measure the time scale of adsorption and interfacial concentrations of these molecules using surface tension as well as to investigate differences between their bulk and interfacial molecular environments (as characterized by molecular orientations and spectroscopic parameters) by comparing the results of bulk UV-Vis and fluorescence spectroscopy and the surface-specific second harmonic generation (SHG) spectroscopy. These studies have begun with several simple PAH's: dibenzofuran, anthracene, and anthracene-9-carboxylic acid. These molecules were chosen for their simple structures and for their solubilities in both aqueous and organic solution. Solutions of all three molecules in hexane have been characterized using UV-Vis and fluorescence spectroscopy, yielding spectra which agree with published work, and which show vibronic structure. From this vibronic structure, the bulk electronic and vibrational environments of these molecules will be able to be compared with those at both the air/water and oil/water interface.

The bulk of the effort on this project has been focused on obtaining reproducible, consistent surface tension measurements at both the air/water and oil/water interfaces (hexane is being used as a model “oil” phase). In all cases, surface tension measurements have been performed using the Wilhelmy plate technique. Measurements at the air/water interface have shown a decrease in surface tension compared to that of neat water, as expected. However, the range of bulk PAH concentrations available for study at this interface is quite limited; while it is energetically favorable for the PAH molecules to be solvated at the interface, the solubility of these molecules in water is extremely small. This results in even very low concentration solutions attaining a minimum surface tension. At this interface, the adsorption time-scale (the time to reach an equilibrium surface configuration, as measured by surface tension) was found to be very short, less than five minutes. At the oil-water interface, developing a procedure which would give consistent, reproducible results has been more challenging, due to both the buried nature of the hexane/water interface and the volatility of hexane. Having now overcome these difficulties, the dibenzofuran and anthracene measurements are now finished, and the anthracene-9-carboxylic acid is being measured. While both dibenzofuran and anthracene show significant surface activity (characterized by a decrease in surface tension as a function of bulk concentration which levels out at a minimum value), their time-scales of adsorption seem to be quite different, with dibenzofuran taking a significantly longer (up to twenty minutes) time to achieve its equilibrium surface configuration. Students are currently applying thermodynamic principles to the data to extract the surface excess from these measurements, which will allow determination of the surface concentration relative to the bulk concentration.

The next step in this work is to take SHG spectra of these molecules, first at the hexane/water interface, because of the larger surface concentrations, and then at the air/water interface. In addition, we will be expanding the number of PAH molecules being studied, to include several which have been functionalized to have higher solubility in water for study at the air/water interface, as well as several larger (more than three aromatic rings) which are essentially only soluble in the oil phase.